N-phosphonomethylglycine, also known by its common name glyphosate, and herbicidal salts thereof are highly effective and commercially important herbicides useful for combating the presence of a wide variety of unwanted vegetation. The herbicide is normally applied systemically or to the foliage of a very broad spectrum of annual and perennial grasses, broadleaf plants and the like. Once within the plant, the herbicide translocates and is lethal to the whole plant when used in an effective amount.
Usually, glyphosate is formulated in herbicidal compositions in the form of a water soluble salt. Commercially, glyphosate in the form of its monoisopropylamine salt is most often sold in concentrated aqueous solution which normally also contains an appreciable amount of a surfactant. The presence of a surfactant which may be ionic or nonionic provides more efficient utilization of glyphosate or its salts dissolved in aqueous solution as compared to like glyphosate-containing solutions to which no surfactant has been added. The more efficient utilization of glyphosate and/or its salts occurs independently of whether the surfactant is anionic, cationic, or nonionic.
When glyphosate-containing herbicides are applied to foliage, as opposed to being directly injected into the plants to be eradicated or controlled, the effectiveness thereof can be severely reduced if rain falls or if water is otherwise applied to the treated plants within a few hours after such application. The severity of the reduced efficacy of the herbicide is directly related to the amount and force of rainfall or applied water and is inversely related to the time that passes between application of the herbicide and the occurrence of rain or other application of water. The commercial label for the herbicide notes that rainfall or irrigation occurring within six hours after application to plants may reduce the effectiveness of the herbicide and that heavy rainfall or irrigation within two hours after application may wash the applied herbicide off the foliage, thereby possibly requiring retreatment of the plants.
Although various ingredients have been suggested for incorporation into aqueous glyphosate-containing solutions either to facilitate penetration of the herbicide into the plant or to hinder, prevent or otherwise restrict the removal of the herbicide from plant foliage by rainfall or other applications of water before the herbicide has entered the plant's systems in a sufficient amount to be lethal to the treated plants, the need for more effective rainfastening additives for glyphosate-containing aqueous solutions continues to exist. As used herein, the term "rainfastness" means the degree of effectiveness of the applied herbicidal composition to be retained notwithstanding a gentle spray of water, especially by rainfall. Improved resistance of glyphosate to being removed from the treated plants or otherwise being rendered less effective by water application is a long-felt need in the art.
Acetylenic diol surfactants have been suggested as an adjuvant for certain herbicides to increase the herbicidal activity via increased phytotoxicity against certain weeds. In a publication by Tedeschi entitled "Surfynol.RTM. Adjuvant-Greenhouse and Field Studies," Proc Northeast Weed Sci. Soc., 33 p 81-82 (1979) it is reported that certain Surfynol adjuvants show promise in increasing the herbicidal activities of atrazine, alachlor, bentazon and chloroxuron. There is no suggestion, as has now been discovered, that any such adjuvants would be useful in enhancing the rainfastness, as well as in enhancing the activity of a glyphosate-containing herbicide applied to plants from an aqueous solution.
It is often desirable to provide glyphosate and any potentiating surfactant in the form of a concentrated aqueous solution. However, it has been found that when acetylenic diol is used as the sole surfactant, a stable homogeneous concentrate formulation is not obtained, except at very low levels of the surfactant and/or at glyphosate salt concentrations around 15% or lower. As one increases the amount of acetylenic diol surfactant up to a point, a corresponding increase in the glyphosate activity will be gained. Unfortunately, when one adds acetylenic diol surfactants to an aqueous solution of glyphosate beyond a certain amount depending on the particular acetylenic diol used, temperature, concentration of glyphosate, etc., it has been found that the surfactant will phase separate, resulting in an unstable, nonhomogeneous formulation. In accordance with the present invention, it has been found that when acetylenic diol surfactants are used, together with at least one other surfactant of certain classes, acceptably stable, homogeneous concentrate glyphosate formulations can be obtained, in some cases also showing improved activity by comparison with formulations containing the acetylenic diol as the sole surfactant. The presence of the second surfactant raises the level of the diol which can be used in concentrates without occurrence of phase separation. Concentrates normally contain glyphosate in an amount requiring a dilution of at least 5 times and up to 100 times with water to have an optimum spray application rate. Suitable concentrates will contain about 5-40 weight percent of glyphosate acid equivalent. Among the second surfactants found useful for this purpose, for example, are included ethoxylated tertiary amines, ethoxylated quaternary amines, propoxylated quaternary amines, alkylglycosides, alkylpolyglycosides, nonalkoxylated amines, ethoxylated amine oxides, nonethoxylated amine oxides, and the like.
Rainfastness of glyphosate-containing aqueous solutions is of particular importance in Southeast Asia, Florida, and similar geographic locations where rainfall often occurs with great frequency and sometimes with considerable force. Often in these areas there is simply not enough time between rainfalls to provide for the highest and best use of glyphosate-containing herbicides. With short time intervals between heavy rainfalls, the herbicide can be washed away or otherwise rendered less useful before it has had time to translocate inside the treated plant so as to be fully effective as a herbicide.
It is known that an ethoxylated siloxane, sold as a tank mix adjuvant under the name Silwet L-77 (herein-after referred to as "L-77" for brevity), provides a limited amount of rainfastness for glyphosate-containing herbicides. L-77 contains as its main active ingredient a member of the silicone-polyethylene copolymer chemical family. More specifically, the active ingredient is polyalkylene oxide modified heptamethyltrisiloxane. Even more specifically, the active ingredient is a 1,1,1,3,5,5,5-heptamethyltrisiloxanyl propyl-omega methoxy-poly(ethylene oxide) where the average number of ethylene oxide units is seven or so.
But, the high cost of L-77, its tendency to form a separate layer in concentrates on standing, thereby requiring stirring to have a homogenous solution, its chemical instability in glyphosate-containing solutions, and its tendency to antagonize glyphosate activity in the absence of rain, limit the use of L-77 for such purpose.
In accordance with the present invention, it has also been found that certain acetylenic diol surfactants when used with a second surfactant provide appreciably improved rainfastness to foliar-applied aqueous solutions of glyphosate and/or its agriculturally acceptable salts. In addition, these diol surfactants often enhance the herbicidal activity of the glyphosate-containing solutions even in the absence of rain. The compositions of the present invention can be formulated as concentrates or tank mixes. The combinations of surfactants are also useful as tank mix adjuvants for aqueous glyphosate-containing solutions.